This invention relates to hydrothermal oxidation processes and to equipment for facilitating hydrothermal oxidation reactions. More particularly, the invention relates to an arrangement for bringing an oxidant into contact with a reactant material to oxidize the reactant material in a hydrothermal process. The invention encompasses both an apparatus and method for bringing the oxidant into contact with the reactant material.
Hydrothermal oxidation involves bringing a reactant material to be oxidized, water, and an oxidant together under an elevated temperature and pressure to effect a partial or complete oxidation of the reactant material. Hydrothermal processes may be carried out at various combinations of temperature and pressure. For example, the reaction temperature may be below the critical temperature for water and the pressure may be below the critical pressure for water. Alternatively, the temperature or pressure, or both may be at or above the respective critical point for water. Although the critical temperature and pressure may vary somewhat depending upon other materials present with water, the critical temperature for water is approximately 705 degrees Fahrenheit and the critical pressure is approximately 3200 pounds per square inch.
Hydrothermal processes may be employed in many different applications. For example, hydrothermal processes, may be used to treat wastewater containing organic and inorganic contaminants. In particular, municipal and industrial sewage sludge may be destroyed using a hydrothermal process to produce primarily heat energy, clean water, carbon dioxide gas, and residual minerals and salts. Heat energy from the hydrothermal process may be used to generate electricity. Also, organic fuels such as coal or petroleum may be oxidized in a hydrothermal process to produce heat energy that can be used for electrical power generation.
Hydrothermal oxidation occurring at conditions above both the critical temperature and pressure for water is commonly referred to as supercritical water oxidation or SCWO. Water at supercritical conditions (SCW) is neither a liquid nor a vapor, but can be properly characterized only as a supercritical fluid having a density significantly less than liquid water but significantly greater than water vapor. The density of SCW increases with increasing pressure at constant temperature. At very high pressures, greater than 40,000 psia for example, the density of SCW resembles that of liquid water. For the purposes of this discussion, SCW is assumed to exist at moderate temperatures between 705xc2x0 F. and 1200xc2x0 F. and moderate pressures of 3200-5000 psia. Nonpolar substances such as oxygen gas and most organic compounds are highly soluble in SCW. Due to the solubility of organic compounds and oxygen in SCW and the characterization of SCW as neither a liquid nor gas, SCW provides essentially a single-phase reaction environment that eliminates the relatively slow process of transferring reactants and products between separate gas and liquid phases. The single-phase reaction environment combined with a high reaction temperature in SCWO results in rapid and complete oxidation of organic compounds. Thus, it is desirable in a hydrothermal process to conduct at least part of the reaction at supercritical conditions in order to rapidly and more completely oxidize the given reactant material.
While nonpolar substances such as oxygen and most organic compounds are highly soluble in SCW, polar substances that may be encountered in hydrothermal processes have very low solubility in SCW. In particular, inorganic compounds such as salts have very limited solubility in SCW even though they may be very soluble in liquid water. Typically, the solubility of salt in water changes by relatively small amounts as the aqueous solution is heated. The solubility change may be seen as a slight increase or decrease in the solubility limit, depending on the specific salt. If the solution is heated to its critical temperature, the solubility of the salt will experience a sudden decrease as the water transitions from a polar solvent to a nonpolar solvent. The largest reduction in salt solubility generally occurs in the near-critical temperature range of 650xc2x0 F. to 720xc2x0 F. The temperature at which a given salt in an aqueous solution begins to experience the sudden decrease in solubility will be referred to in this disclosure and the accompanying claims as the xe2x80x9csalt precipitation temperature.xe2x80x9d
Although the solvent properties of SCW are very desirable in destroying organic compounds, the low solubility of inorganic salts in SCW has posed problems in prior SCWO systems. Salts may enter a SCWO system as part of the feed stream being treated or may form later in the reaction stream as a result of hydrolysis and the oxidation of organic heteroatoms such as sulfur, phosphorus, and nitrogen. Regardless of the source of the salts in the SCWO system, the salts precipitate from the reaction stream as the salt precipitation temperature of the solution is approached. The precipitated salts adhere to the internal surfaces of devices in the SCWO system to form scale. These scale deposits may occur in heat exchangers, heater coils, and reactors in a SCWO system, resulting in reduced heat exchange capacity, increased back pressure within the system, and ultimately, a completely plugged system. Thus, SCWO systems must be shut down periodically to remove scale deposits and thereby restore heat transfer efficiency and prevent plugging.
Numerous solutions have been proposed to overcome the salt scaling problem in hydrothermal processes. Some proposals include arrangements that treat rapid scale formation as an inevitability, and simply address the cleaning process. Other proposed solutions involve protecting the walls of the hydrothermal reactor to prevent deposition of precipitated materials. One of these wall-protecting solutions is described in U.S. Pat. No. 5,670,040, and involves conducting the supercritical oxidation reaction in a special platelet or transpiration tube. This transpiration tube includes openings that allow water to be continuously injected into the tube. The injected water is intended to form a protective barrier at the surface of the tube in order to prevent precipitated materials from adhering to the tube.
The previously proposed solutions to the scaling problem in hydrothermal processes, including the above described proposals, have generally proven unacceptable for various reasons. Most of the proposed solutions are costly and do not adapt themselves well to a continuously operating, robust system. Others simply do not work at the demanding conditions required for supercritical water oxidation. Considering the desirable attributes of supercritical water oxidation for waste treatment and other applications, there remains a need for a solution to the problem of rapid salt scaling in supercritical water oxidation systems.
It is an object of the invention to provide an apparatus and method to overcome the above-described problems associated with hydrothermal oxidation processes. More to particularly, it is an object of the invention to provide an apparatus and method for reducing or eliminating salt deposition (scaling) in hydrothermal oxidation reactors, heat exchangers, heaters, and related equipment.
This object is accomplished according to the invention by controlling the spatial location and temperature and pressure conditions where an oxidant comes into contact with reactant material and salts are formed and/or precipitated in the hydrothermal system.
As used in this disclosure and the accompanying claims, the term xe2x80x9chydrothermal oxidationxe2x80x9d means an oxidation reaction in the presence of water at an elevated temperature and pressure. The term xe2x80x9creactant materialxe2x80x9d means the feed material to be treated in the hydrothermal oxidation process, and may include water along with materials to be oxidized. xe2x80x9cOxidantxe2x80x9d refers to any oxidant material suitable for use in the hydrothermal oxidation process including, air, oxygen, hydrogen peroxide, or nitrate, for example. The term xe2x80x9creaction streamxe2x80x9d means the stream of materials existing after oxidant, water, and reactant material are combined in the hydrothermal system.
According to the present invention, pre-heated and pressurized reactant material is caused to come into contact with oxidant in an initial contact zone within a contactor vessel. This initial contact zone is removed from the walls of the vessel and the structure through which the reactant material is introduced into the vessel. The temperature of the reactant material is elevated to a temperature greater than the salt precipitation temperature for the expected salts in at least portions. of the initial contact zone. Sulfates and other negatively-charged ions formed by oxidation of reactant material in the initial contact zone associate with calcium and other positively-charged ions to form salts. Sulfate, calcium and other ions introduced with the reactant material will also associate to form salts in the initial contact zone. if Because the temperature within portions of the initial contact zone in the contactor is greater than the salt precipitation temperatures of the various salts in the initial contact zone, the majority of salts will precipitate from the solution generally in the initial contact zone removed from the contactor vessel walls. These precipitated salts then tend not to adhere to equipment at points downstream from the initial contact zone. Thus, the precipitation of salts in the contactor vessel at points removed from the contactor vessel walls reduces or eliminates scale buildup on the contactor vessel walls and in equipment downstream from the contactor vessel.
The method according to the invention includes injecting reactant material into the initial contact zone within the contactor vessel. This injected reactant material has previously been pressurized to a processing pressure above the critical pressure and heated to an elevated temperature which may be below the critical temperature and preferably below the reactant material charring temperature. For purposes of this disclosure and the accompanying claims xe2x80x9ccharring temperaturexe2x80x9d for a given reactant material is defined as the temperature at which the rate of formation of thermal decomposition products (char) in the reactant material results in unacceptable fouling of heat transfer surfaces in the hydrothermal treatment system. The method further includes injecting a separation material, preferably water, into a separation zone adjacent to the reactant material in the initial contact zone. This injected water has been pressurized to the processing pressure and typically heated to a temperature higher than the reactant material temperature, near or above the critical temperature for water. The method also includes injecting the oxidant into the contactor vessel at one or more points separated from the reactant material by the water in the separation zone.
In operation, the oxidant diffuses quickly through the xe2x80x9cinjectedxe2x80x9d water at or near critical conditions. Once the oxidant has diffused through the injected water and reaches the reactant material in the initial contact zone, the oxidation of reactant material progresses rapidly to form various reaction products. Salts in or near the initial contact zone precipitate from the aqueous-fluid phase generally in or near the initial contact zone and in any event away from the walls of the contactor vessel. It has been found that this control of the location and conditions under which the oxidant comes into contact with the reactant material according to the present invention, together with the appropriate fluid velocity of the reaction stream, prevents the precipitated salts from adhering substantially to the contactor vessel walls or to the walls of equipment downstream from the initial contact zone.
The step of injecting the reactant material into the contactor vessel preferably includes injecting the reactant material into the vessel through a reactant material injection area centered on and perpendicular to a longitudinal axis of the contactor vessel. In this preferred form of the invention, water is injected into the contactor vessel through an annular area around the reactant injection area and oxidant is injected through an annular oxidant injection area around the water injection area. In order to ensure that the oxidant contact with the reactant material does not occur until the streams reach points downstream from the injection areas themselves, the reactant injection area, water,injection area, and oxidant injection area are all aligned in a common injection plane. Also, the reactant material, water, and oxidant are each preferably injected into the contactor vessel to provide a generally laminar flow regime in the water and reactant material for a substantial distance downstream from the injection plane. The laminar flow regime in the water and reactant material helps prevent the two materials from mixing too rapidly and placing reactant material in initial contact with oxidant near the contactor vessel walls or material injection structures.
The preferred form of the invention includes heating the reactant material and water prior to injection into the contactor vessel by heat exchange with effluent from the hydrothermal oxidation reaction. Since the reactant material and water are preferably heated to different temperatures, the method preferably includes splitting the hydrothermal oxidation effluent stream in at least two separate streams. This split may be accomplished at supercritical conditions with a hydrocyclone to form a suspended solids-laden underflow and clean fluid overflow. One effluent stream may then be placed in a heat exchange relationship with either the reactant material or water, and the other effluent stream is placed in a heat a exchange relationship with the remaining material. In the preferred form of the invention, the water is heated to a higher temperature than that of the reactant material. In this way, not only does the water serve to initially separate the oxygen and the reactant material, but it also provides rapid heating of the reactant stream to a temperature greater than the salt precipitation temperature. The oxidant begins to react with the reactant material at substantially the same time that salt precipitation temperature is achieved in the reactant stream, now at least partially mixed with the injected water.
The apparatus according to the invention includes a contactor vessel having inner walls defining a contactor volume encompassing the initial contact zone spaced apart from the vessel walls. A reactant material injector and an oxidant injection arrangement are both connected to the contactor vessel. The reactant material injector is connected to the contactor vessel in a position to inject the reactant material into the initial contact zone, while the oxidant injection arrangement includes one or more oxidant injection openings at points spaced apart from the reactant material injector by a separation material or water injection area.
The preferred contactor apparatus includes a generally cylindrical contactor vessel having a longitudinal axis defining a contactor axis. The reactant material injector includes a reactant material conduit coaxially mounted in the contactor vessel with an end of the conduit defining a reactant material injection area. A water conduit has an inner diameter larger than the reactant material conduit and is mounted coaxially with the reactant material conduit. The annular area between the water conduit and the reactant conduit comprises the water injection area in the preferred form of the invention. A still larger diameter, coaxially mounted conduit defines an annular area outside of the water injection area and this annular area comprises the preferred oxidant injection opening.
These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.